1. Field of the Invention
This invention relates to an image forming method operating on thermal transfer technology and an apparatus for forming images by the method.
2. Description of the Related Art
Thermal transfer technology is one of the conventional methods of forming images on transfer media such as paper and films. In this technology, a transfer medium (toner sheet) comprising a base having on its surface a transfer layer of pigment-base toner particles which are a transfer material and a receptor (image-receiving sheet) such as paper or a film are placed one on the other and heat is applied image-wiseimage-wise to the back of the toner sheet""s base by means of a thermal head, a laser head or the like so that the toner particles are thermally transferred onto the image-receiving sheet to form an image on it. One way to implement the thermal transfer technology is dye-sublimation in which colored dyes are converted from a solid state to vapors and then transferred onto an image-receiving sheet, where the vapors cool back to solid colors.
Images can also be formed on an image-receiving sheet by ink-jet technology where ink is forced out in drops. The essential components of an ink-jet printer are a plurality of inkhead""s nozzles, mutually independent firing chambers that communicate with the orifices of the nozzles and which have flexing walls, piezoelectric devices mounted on the flexing walls, and a common ink cavity from which ink is supplied into the firing chambers.
When a pulsed voltage associated with image information is applied to the piezoelectric devices, the walls of each firing chamber flex mechanically to reduce its volume, whereupon the internal pressure is increased rapidly enough that ink drops are successively propelled through the nozzle to form an image on the image-receiving sheet.
The use of a thermal head in the thermal transfer technology involves the problem that due to various factors including the variations in the resistances of heat generating elements on the head, nonuniform contact between the head and the transfer medium and unevenness in the thickness of the base of the thermal transfer medium, there is a high likelihood for uneven temperature profile and unevenness occurs in the image being formed. As a further problem, localized heating at high temperature increases the chance of the base deforming to produce wrinkles.
The use of laser light involves not only substantial investment cost but also a high likelihood for the occurrence of image unevenness due principally to the decomposition of materials upon momentary heating at elevated temperature.
The ink-jet technology is free from the problem of base deformation due to the heat being applied during recording but it is mandatory to prevent the clogging of nozzles by ink so that uniform ink drops will form consistently. This poses many limitations on the choice of image forming materials in terms of liquid""s physical properties and color imagery is not an exception. In order to prevent the clogging of nozzles, ink materials have to be chosen from a limited list of dyes and pigments and colors identical to those of printing ink pigments cannot be reproduced; this has made the ink-jet technology unsuitable for use on high-precision proofing printers. As further problems, ink-jet formed images do not have high lightfastness and waterfastness and the dyes tend to bleed on the receptor, eventually causing failure to produce high-resolution images.
With a view to solving these problems, the present inventors previously proposed a technique in which a liquid substance capable of lowering the transfer temperature was propelled imagewiseimage-wise onto a transfer material for use in the conventional heat transfer technology and hot transfer was subsequently performed to produce a uniform image of high color reproduction fidelity while ensuring against the clogging of nozzles. This proposal was described in Japanese Patent Application No. 170702/1999.
This approach can produce transferred images of high quality. However, it is apprehended that when transfer is repeated plural times to obtain a multi-color image, a color can be easily transferred onto the image which has been just transferred, causing the occurrence of so-called secondary color (multi-order color) sensitization, i.e., staining of the transferred color image.
In order to use this approach for the preparation of print proof for proving the printing conditions, it is necessary to transfer an image onto a dedicated image-receiving sheet from which it is then transferred onto a printing sheet. It has thus been desired to realize a process for transferring an image directly from the transfer layer onto the printing sheet for the purpose of simplifying the process and improving the image quality of print proof. The term xe2x80x9cprinting sheetxe2x80x9d as used herein is meant to indicate paper, film, metal foil or other materials on which printing is effected.
This approach could produce transferred images of high quality. However, it was suspected that when the liquid substance was passed between hot pressure rolls after being forced out of the nozzles, the transferred dots might spread depending upon the temperature and pressure conditions, thus lowering the resolution of the image being formed. In order to prevent the decrease of resolution, precise temperature and pressure adjustments have to be made but this complicates rather than simplifies the adjustment procedure.
The present invention has been accomplished under these circumstances and has as an object solving the problems of limited image forming materials and clogged nozzles which are posed by the ink-jet technology while eliminating the two drawbacks of the thermal transfer technology, a damaged base and an uneven image, as well as attaining the following advantages:
1) A transferred image can be directly formed on the printing sheet;
2) A transferred image can be formed free from secondary color (multi-order color) sensitization;
3) image forming materials difficult to use in the conventional ink-jet technology can be employed to record images;
4) uneven images can be formed at low cost without using an expensive laser head;
5) images can be formed without damaging or wrinkling the base;
6) highly lightfast and waterfast images can be formed without bleeds; and
7) high-resolution images can be formed.
The stated object of the invention can be attained by the image forming method as one of the embodiments of this invention which uses a transfer medium having a transfer recording layer on a base and a receptor onto which said transfer recording layer is to be transferred, characterized by comprising:
a latent image forming step in which in order to transfer said transfer recording layer onto the surface of said receptor, a solution that lowers the transfer temperature of said transfer recording layer or a sticky solution is applied image-wiseimage-wise to the surface of either said transfer recording layer or said receptor so as to form a latent image;
a thermal drying step in which at least a portion of the solvent contained in the formed latent image is evaporated by heating; and
a transfer step in which said transfer medium and said receptor are heated, with the surface of the transfer recording layer in intimate contact with that of the receptor, so that said transfer recording layer in areas corresponding to the ones where said latent image has been formed is transferred onto the receptor.
In the latent image forming step of the image forming method, a solution that lowers the transfer temperature of the transfer recording layer or a sticky solution is applied imagewiseimage-wise to the surface of either the transfer recording layer or the receptor so as to form a latent image; in the subsequent thermal drying, at least a portion of the solvent contained in the formed latent image is evaporated by heating; and in the following transfer step, the transfer medium and the receptor are heated, with the surface of the transfer recording layer in intimate contact with that of the receptor, so that the transfer recording layer in areas corresponding to the ones where the latent image has been formed is transferred onto the receptor. As a result, when the transfer medium is brought into intimate contact with the receptor, the already formed latent image will not be transferred as larger dots and, instead, the transfer recording layer will be directly formed on the receptor at high resolution. This eventually improves image resolution, simplifies the image forming process and enables the formation of even images at low cost.
The image forming method also solves the problem of uneven temperature profile which often occurs in the conventional thermal transfer technology due to various factors such as unevenness in the thickness of the base, uneven contact with the thermal head and uneven heat diffusion. The method is also free from the problems of limited choice of image forming materials and the clogging of nozzles that are frequent in the conventional ink-jet technology.
The stated object of the invention can also be attained by the image forming apparatus as another embodiments of this invention which performs image-wiseimage-wise transfer onto a receptor of a transfer recording layer in a transfer medium which is provided on a base and which is made of a thermal transfer material, characterized by comprising:
a drop firing head with which in order to transfer said transfer recording layer onto the surface of said receptor, a solution that lowers the transfer temperature of said transfer recording layer or a sticky solution is applied image-wise to the surface of either said transfer recording layer or said receptor so as to form a latent image;
a thermal drying means which heats either said transfer medium or said receptor or both on which said latent image has been formed so as to evaporate at least a portion of the solvent contained in the formed latent image; and
a pair of pressure rollers which hold said transfer medium and said receptor together, with the surface of the transfer recording layer in intimate contact with that of the receptor, at least one of said pressure rollers having a heating means capable of controlling the heating temperature.
By means of the drop firing head in the image forming apparatus, a solution that lowers the transfer temperature of the transfer recording layer or a sticky solution is applied image-wise to the surface of either the transfer recording layer or the receptor so as to form a latent image; by the thermal drying means, at least a portion of the solvent contained in the formed latent image is evaporated by heating; and by means of the pair of pressure rollers having a heating means, the transfer medium and the receptor are heated, with the surface of the transfer recording layer in intimate contact with that of the receptor, so that only the transfer recording layer in areas corresponding to the ones where the latent image has been formed is transferred onto the receptor. As a result, when the transfer medium is brought into intimate contact with the receptor, the already formed latent image will not be transferred as larger dots and, instead, the transfer recording layer will be directly formed on the receptor at high resolution. This eventually improves image resolution, simplifies the image forming process requiring only a straightforward structural design and enables the formation of even images at low cost.
Said image forming apparatus may be also modified such that said thermal drying means has a heating temperature control unit for setting the heating temperature in accordance with the result of transfer of the transfer recording layer.
Since the thermal drying means controls the setting of heating temperature in accordance with the result of transfer of the transfer recording layer, image can be formed at an optimum transfer temperature to maintain high image quality throughout the service of the apparatus.